How do I fix a rocking paving slab?
Why a slab rocks even when it looks level, the quick stabilisation that is worth doing, and the point at which it has to come up.
- Difficulty
- beginner
- Time
- 1 hr
- Read
- 3 min
Short answer
A rocking slab has a void under it, usually because it was spot-bedded and the spots have failed. The only real fix is to lift it and rebed it on a full mortar bed. Injecting sand or grout under it is a short-term measure at best.
A rocking slab is a trip hazard and it gets worse, because every rock pumps water and bedding material out from underneath. It is also the earliest warning that the patio was spot-bedded, which means the others will follow.
Step by step
- Find out how it moves.Stand on each corner in turn. A slab that pivots diagonally has a void in the middle or at one corner. A slab that drops uniformly has a base problem, not a bedding one.
- Check the neighbours.Tap across the area with a mallet handle. A hollow ring means a void underneath. If several slabs sound hollow, the whole patio was spot-bedded and will need progressive attention.
- Mark it as a hazard if you cannot fix it today.A rocking slab on a path or by a door is a genuine trip risk, especially for anyone unsteady on their feet. Cone it, tape it, or take it out and fill the hole temporarily.
- Rake the joints out around the slab.All four sides, to full depth, before levering. Levering against a mortared joint breaks the adjacent slab.
- Lift the slab.Bolster in the joint to start it, then a bar underneath. Two people or slab lifters for anything over 450mm square.
- Clear out and inspect the base.Remove all old bedding. Push a screwdriver into the base — if it goes in easily, the sub-base has failed and needs excavating and rebuilding with compacted MOT Type 1.
- Rebed on a full mortar bed.Semi-dry 4:1 sharp sand and cement, 30-50mm thick, covering the whole slab footprint. Lay the slab, tap it down with a rubber mallet, and check with a straightedge across three slabs.
- Leave and then repoint.24 hours before walking on it, then point the joints to match the rest of the patio.
Tips
- If more than a couple of slabs rock, budget to lift and rebed a whole area rather than chasing them individually.
- Check whether a downpipe or a leaking outside tap is washing bedding out. Fixing that first stops the problem repeating.
- A slab that rocks only in winter often has a void that fills with water and freezes. It is the same fix.
Common mistakes
- Brushing sand into the joints to stop the rock — It does nothing for the void underneath, which is what is actually moving. The slab still rocks and now the joint is full.
- Packing a shim under one corner — It concentrates the load on a point and the slab cracks. Support has to be under the whole slab.
If it doesn't work
Slab rocks, no visible level difference
Cause: Void under the middle from spot bedding — Fix: Lift and rebed on a full mortar bed. The level being right is irrelevant if there is nothing supporting the middle.
Slab cracked across the middle
Cause: Loaded over a void — Fix: Replace the slab and rebed fully. A cracked slab will not survive being relaid.
Several slabs hollow-sounding across the patio
Cause: The whole patio was spot-bedded — Fix: Plan to lift and rebed area by area. Prioritise anywhere people walk regularly or where a trip would matter.
Slab settles again after rebedding
Cause: Sub-base failure below the bedding — Fix: Excavate to firm ground and rebuild the sub-base in compacted layers before rebedding.
Questions people ask
Can I fix a rocking slab without lifting it?
There are injection grouts sold for stabilising slabs and they can work as a temporary measure. They do not address a failed sub-base, and on a patio that was spot-bedded throughout you are treating symptoms.
Why do so many patios have rocking slabs?
Spot bedding — five blobs of mortar under each slab — was standard practice for years. It leaves voids that fill with water, freeze and break down, so the slabs start rocking within a few years.